Because in their battle, Anakin went to the Sith way but hadn't learned how to control his anger and hate. So his arrogance caused his defeat.
Because he is THE master of Sosoru, which is a defensive stance which pretty much he wears down his opponent until they slip up and make a mistake. Though in most of his fights, Obi-Wan took a rather big gamble.
It's quite useful against Anakin's Djem So, but no match for Dooku's Makashi.